Sine-cosine synchro resolver circuit arrangements



Nov. 28, 1961 A. B. STARKS-FIELD ETAL 3,011,119

smz-cosmz SYNCHRO RESOLVER CIRCUIT ARRANGEMENTS Filed Dec. 29, 1958 INVENTORS I This application is a continuation-in-part of application Serial No. 599,651, filed July 23, 1956, now abandoned.

This invention relates to sine-cosine synchro resolver or angular motion transmission arrangements generally known as selsyn systems, and has for its object to provide such arrangements having improved linearity of response.

It is often required for a number of technical purposes, notably for actuating a P.P.I. radar displayemploying a cathode ray tube with mutually perpendicular fixed deflecting coils, to resolve the waveform from a master time base into two components, namely a sine component and a cosine component, in order to produce the required rotating time base for the display. One well known way of doing this is to employ a synchro resolver or angular motion transmission arrangement or selsyn device of the kind having an input winding on one of the relatively rotatable members, and two output windings located at 90 to one another on the other relatively rotatable member, each member having a magnetic core, the arrangement being such that when an input signal is applied, two

output signals proportional respectively to the sine and cosine of the angle between the rotatable members are developed. By rotating the synchro resolver rotor or selsyn device at a desired speed and applying the two output voltages to the two fixed mutually perpendicular deflection coils of a cathode ray tube, a rotating timebase is obtained. It is, of course, well known, e.g. from British Patent No. 579,274, to obtain improved linearity by using a synchro resolver or selsyn device of the type having a feedback winding tightly coupled to the main winding. In this type of known arrangement, the main input winding of the resolver or selsyn device is fed through an amplifier and linearizing, negative, feedback is applied to the input of the amplifier from the feedback winding. In order to ensure that the quadrature magnetic field in the resolver or selsyn device shall be zero, it is also customary to provide the device with a quadrature field correction circuit arrangement inthe form of a second main winding and a second feedback winding on the member carrying the input winding, and

arranged at 90 to the main input winding, said quadrature correction windings being connected in a -similar manner to the main input windings but having the amplifier input earthed. Such a synchro resolver or selsyn device, although ditferently connected, is known from the output from the input to the time base amplitude, expressed as a percentage, is 0.1%, so that, for a time base of, say, 50 volts amplitude, the maximum deviation will be 50 millivolts. The present invention seeks to provide substantially improved linearity and enable linearity of the order of live times as good as the foregoing to be obtained, i.e. to enable linearity of the order of 0.02% or less to be obtained. This lack of good linearity of the output waveform is believed to be due to the nonlinear hysteresis curve of the core material and to the lack of tight magnetic coupling between the windings on the rotor and stator, respectively.

It has been found that the remaining non-linearity may with time of the magnetizing force in said resolver or selsyn device, and a second voltage, which is substantially proportional to the rate of change with time of the flux density in said resolver or selsyn device, in addition to the linearizing feedback referred to hereinbefore.

Preferably the first voltage is obtained by differentiating a voltage substantially proportional to the current flow through the main input-winding of the resolver or selsyn device and the second voltage is obtained from across the feedback winding of said resolver or selsyn device. In the simplest case the said first voltage is derived by ditferentiating the voltage set up across a re sistance in series with the main input winding.

Preferably the first and second voltages are combined in a mixing amplifier, at least one of the two voltage inputs thereto being taken via an adjustable potentiometer across which said voltage is applied. a

The mixing amplifier may conveniently be a difference amplifier comprising two valves, the grid of one of which receives the first voltage input and the grid of the other of which receivesthe second voltage input, the combined output being taken from the anode of one of these valves. Such a diiferenceamplifier is known from Pulse and Digital Circuits, by Millman and Taub, published by McGraw-Hill & Co., page 20, FIGS. 1-16.

The invention is illustrated in the accompanying draw-f irig, which shows diagrammatically one embodiment there- 0 Referring to the drawing, the apparatus therein shown includes a synchro resolver or selsyn device of the known type having feedback and quadrature field correction windings. This synchro resolver or selsyn device comprises mutually perpendicular rotor windings 1 and 2, a main stator winding 3, a feedback winding 4 tightly coupledto the winding 3, and at right angles to the Windings 3' and 4, a corresponding pair of closely coupled quadrature field correction windings 5 and 6. Sine and cosine component outputs are taken from the terminals 7 and 8 respectively.

7 The wa'veform to beresolved into sine and cosine components assumed to be a normalsaw-tooth wave form, such as is illustrated adjacent terminal 11,-is fed fromterminal 11, via an AC. coupling condenser to an Operational Amplifier of the kind described in Pulse and Digital Circuits, supra, onpa'ges 22 and 23,.FIGSV 1718, and an impedance comprising resistor 24 and condenser 25 is connected in parallel to the grid of the first valve 26 of a high gain D.C. amplifier comprising the valves 26 and Z7 and their associated circuits. Such Patented Nov. 28, 1961 D.C. amplifiers are well known and the normal connections thereto are indicated in conventional manner. The output at the anode of valve 26 is fed to the grid of valve 27 via a potentiometer arrangement, the condenser connected across the upper arm of the potentiometer being provided in accordance with well known practice to give high frequency phase compensation. The cathode of valve 27 is connected to the main stator Winding 3 of the synchro resolver or selsyn device, the other end of the winding 3 being connected to earth through the resistor 13. The A.C. voltage induced in the feedback winding 4 is fed from one end of this winding via the feedback resistor 20' to the grid of the valve 26 as negative, linearizing, feedback. In addition D.C. feedback is provided from the main input winding 3 via the resistor 22 in series with the A.C. feedback in order to control the valve currents of the D.C. amplifier, and condenser 21 is a de-coupling condenser connected between the other end of the feedback winding and earth. The use of such D.C. feedback in DC. amplifiers is well known and the requirements therefor are dealt with in the M.I.T. Radiation Laboratory Series, in vol. XVIII, page 467 (FIG. 11-51). As is well known in connection with amplifiers as described above, the overall gain is a function of the ratio of the impedance in the feedback path, constituted mainly by the resistor 20 and the condenser 21, to the impedance in the input path, constituted mainly by the resistor 24 and the condenser 25.

The quadrature field correction circuit including a high gain amplifier comprising the valves 28 and 29, feeding the main quadrature winding and fed from the feedback quadrature winding 6, operates in a similar manner to the amplifier described above, but in this case the input terminal 1% of the amplifier is earthed. Thus if any undesired quadrature field is present in the synchro resolver or selsyn device, the voltage induced in the feedback quadrature winding is applied to the control grid of the valve 28 in such a way as to produce an output in the main quadrature winding 5 to cancel the quadrature field.

In series with the said main coil 3 is a resistance 13, across which is set up a voltage proportoinal to the current through the said main coil. This voltage is difierentiated by a differentiating circuit comprising a condenser 14 in conjunction with the input impedance of a difference amplifier consisting of the valves 15 and 16 and connected in manner well known per se. The differentiated voltage is applied to the control grid of the valve 15 and the second input to the differential amplifier is applied to the valve 16 and is derived from an adjustable potentiometer 17 connected across the feedback coil 4.

As it is desired that the output from the difference amplifier should be the difference between the aforesaid differentiated voltage and the voltage developed across the feedback winding 4, and as the input to the valve 16 is the sum of the voltage across the winding 4 and the voltage across the condenser 21, it is necessary to add this latter voltage to the differentiated voltage. This is done by connecting the end of the condenser 21 remote from earth to the grid of valve '15. For simplicity, the waveforms shown adjacent the lines from the potentiometer 1'7 and the main winding 3 to the difference amplifier are shown without the voltage component existing across the condenser 21. The output from the difference amplifier is taken from the anode of the valve 15, ampli fied by a cathode follower including the valve 23 whose output waveform is shown adjacent the lead from the cathode follower load of valve 23 to the resistance 18 and applied through an adjustable resistance 18'which controls the total feedback from the valve 23 and A.C. coupling condenser 19 as linearizing feedback to the amplifier 26. The signal at the anode of valve 15 is fed to the grid of valve 23 via a potentiometer arrangement,

A} the condenser 32 and the network 33 providing high frequency phase compensation in well known manner.

To reduce the overall gain of the amplifier to a desired value and to increase its stability and linearity, negative feedback is applied from the output of the cathode follower valve 23 to the grid resistor of the valve 15.

The amplifier arrangement including the valves 15, 16 and 23 is thus seen to be an operational amplifier of the kind described in Pulse and Digital Circuits by Millman and Taub on pages 212 to 23 and referred to above in connection with the amplifier including valves 26 and 27. In the amplifier including the valves 15, 16 and 23 the effective impedance in the input path is constituted mainly by the impedance network 3'9 and the impedance in the feedback path is constituted by the impedance network 31.

By suitable adjustment of the potentiometer 17 which determines the relative amplitudes of the two input signals to the difference amplifier constituted by valves 15 and 16 and the resistance 18 which determines the amplitude of the correcting feedback applied to the grid of valve 26, a very high degree of linearity can be obtained. In practice the linearity of the output waveform from one of the rotor windings 1 and 2 of the synchroresolver is observed and the potentiometer '17 and variable resistance 18 are adjusted until good output linearity is obtained. As will be seen, one input to the differential amplifier 1516 is substantially proportional to the rate of change with time of the flux density in the synchro resolver or selsyn device, while the other is substantially proportional to the rate of change with time of the magnetizing force in the synchro resolver or selsyn device, the combination of these two inputs in correct proportion providing the high degree of linearity obtained.

While we have described our invention in one of its preferred embodiments, we realize that modifications may be made and we desire that it be understood that no limitations upon our invention are intended other than may be imposed by the scope of the appended claims.

What we claim as new and desire to secure by Letters Patent of the United States is as follows:

1. A sine-cosine synchro resolver arrangement including a synchro resolver of the kind having two magnetically cored relatively rotatable members with an input winding and a feedback winding tightly coupled to said input winding on one of said members, and two output windings located at to one another on the other of said members, the arrangement being such that the output signals derived therefrom are proportional respectively to the sine and cosine of an angle between said two relatively rotatable members; an amplifier fed with the waveform to be resolved into sine and cosine components and feeding said main input winding, means for applying to the input of said amplifier as negative feedback a voltage developed across said feedback winding, means for deriving a first voltage substantially proportional to the rate of change with time of the magnetizing force in said resolver, means for deriving a second voltage substantially proportional to the rate of change with time of the flux density in said resolver and means for deriving a difference voltage proportional to the difference between said first voltage and said second voltage, said difference voltage being fed as. linear izing feedback to the input of said amplifier.

2. An arrangement as set forth in claim'l wherein said' synchro resolver includes two tightly coupled quadrature field correction windings disposed at right angles to said input winding and wherein is provided an amplifier fed by one of said quadrature field correction windings and feeding the other of said quadrature field correction windings in such manner as effectively to cancel any quadrature field. Z l V 3. An arrangement asset forth in claim 1 wherein said means for deriving a first voltage comprises means for deriving a voltage substantiallyproportional to the cur rent in said input winding and a difierentiating circuit fed with said voltage and wherein said means for deriving a second voltage comprises a resistance connected across said feedback winding.

4. An arrangement as set forth in claim 3 wherein said means for deriving a first voltage substantially proportional to the current in said input winding comprises a resistance in series with said input winding.

5. An arrangement as set forth in claim 1 wherein said means for deriving a difierence voltage comprises a difference amplifier having two input points, and an adjnstable potentiometer interposed between said feedback winding and one of said input points.

References Cited in the file of this patent UNITED STATES PATENTS 

